Myalgic encephalitis/chronic fatigue syndrome (ME/CFS) is an illness characterized by profound fatigue lasting at least 6 months and accompanied by numerous somatic symptoms. Infectious, immunological, neuroendocrine, sleep, and psychological mechanisms for ME/CFS have been investigated but a unifying etiology has yet to emerge. We hypothesize that peripheral tissue abnormalities significantly contribute to ME/CFS pathogenesis. Amongst other features, patients with ME/CFS have substantial abnormalities of intramuscular metabolites following a standardized exercise challenge and metabolite efflux from ME/CFS muscles is significantly lower than in NC immediately post-exercise. We propose that accumulation of these metabolites results in stimulation of ERGOreceptors (ERGO) (afferents sensitive to muscle metabolites) and nociceptors in muscles with resulting autonomic nervous system (ANS), HPAaxis, fatigue, and pain pathway activation. Excessive activation of these pathways then leads to characteristic ME/CFS symptoms including profound fatigue and often pain. We hypothesize that these pathways have become sensitized in ME/CFS resulting in amplification of peripheral signals to the CNS. We will characterize ME/CFS subjects' peripheral contributions to fatigue and pain including sensitized ERGO and nociceptor pathways using the post-handgrip exercise regional circulatory occlusion (PH-RCO) method and quantitative sensory testing (QST), including mechanical and heat thresholds. PH-RCO traps exercise related metabolites in muscles post exercise allowing estimates of peripheral contributions to fatigue and pain. This results in prolonged activation of ERGO and pain receptors [3] and subsequent post-exertional fatigue and pain. In order to test the relevance of ERGOs for chronic fatigue and pain we propose reducing ERGO signaling in muscles of ME/CFS patients. For this purpose we will use placebo controlled lidocaine injections into several muscle groups to reduce ERGO activity and thus ongoing fatigue and pain of ME/CFS patients. This approach holds great promise because our preliminary data have shown that such injections can result in robust reductions of fatigue and pain (Cohen's d >1.0). Thus therapeutic muscle injections may represent a clinically relevant model to study the contributions of ERGO induced fatigue and pain to ME/CFS. To estimate treatment effects on both overall fatigue and pain in ME/CFS subjects, we will utilize validated fatigue and pain assessments, including several fatigue scales and QST. Thus we will characterize overall and local effects of muscle injections on ME/CFS subjects' fatigue and pain abnormalities. In order to assess the neural correlates to fatigue and pain in ME/CFS subjects we will perform functional brain imaging (fMRI) using arterial spin labeling (ASL). We will also utilize fMRI to characterize the cerebral blood flow changes of PH-RCO related fatigue in ME/CFS compared to NC subjects. In addition, we will perform neural connectivity analyses of resting state networks before and after PHRCO which will characterize connectivity changes induced by fatigue. Overall, our proposal will help characterize peripheral and central nervous system contributions to chronic fatigue and pain in patients with ME/CFS that may lead to novel therapies for this chronic illness.